The phrase "most buttoned down" typically refers to someone or something that is very conservative, traditional, and strictly adheres to rules or protocols. Some suitable antonyms for this expression could be "laid-back," "uninhibited," "free-spirited," "casual" or "relaxed." These words suggest a liberal, easy-going and informal approach that emphasizes individuality, spontaneity and creativity over strict conformity or formality. For example, a person who is "laid-back" might be perceived as careless, while "free-spirited" indicates someone who is not afraid to take risks or push boundaries. In short, antonyms for "most buttoned down" are words that convey a sense of openness or unconventionality.